Corrosion is a serious problem that affects and undermines the quality of everyday social life and the endurance of industrial products. Extensive efforts have been undertaken to design and fabricate products such as steam generators, heat exchangers, bridges, oil drilling platforms and motor vehicles that can resist the destructive effects of corrosion. For many traditional products, however, corrosion continues to be a serious destructive problem. In addition to traditional products, new developments in energy sources, advances in material sciences, microprocessing technology and miniaturization of new devices to the micron scale all reassert the demands for effective anticorrosion technologies that avert device failures.
Corrosion is typically understood as an electrochemical reaction that involves loss of electrons from metals, a reaction more generally described as oxidation. The definition of oxidation in electrochemical terms is independent of whether or not oxygen is present when the process of electron loss from a metal occurs. The loss of one or more electrons from a metal requires the acquisition of the electron(s) by another agent. Thus, the metal that serves as an electron donor in this case is termed a reducing agent, while the electron acceptor serves as an oxidizing agent.
One practical example illustrating such an electrochemical event typically involves metallic iron. Electrons lost from an iron atom (reducing agent) can be acquired by oxygen (oxidizing agent) to produce a new combined iron and oxygen derivative identified as iron oxide, or rust, which is an inorganic, low density (flaky) product commonly associated with metal corrosion. Although oxygen is used as a model of iron oxidation in this case, the same oxidizing agent effect could be demonstrated by sulfur and the resulting product could have been iron sulfide instead of iron oxide.
Apart from rust involving metal corrosion, the formation of scale presents another illustrative model tied to principles of oxidation. Scale is defined as a thin coating, layer or encrustation of material that is rich in complex oxides of sulfur, magnesium and/or calcium. These and other insoluble materials are typically developed and observed as mineral deposits on the inside diameters of pipes, chambers or containment vessels when water plus its dissolved constituents, or solutes, are heated in the process of making hot water.
The transfer of electrons between oxidizing agents and reducing agents cannot occur without the presence of an electrically conductive medium. Water typically serves as the electrically conductive universal solvent medium that supports metal oxidation, consequential corrosion and rusting as well as scale formation in the foregoing models.
Efforts to halt water-mediated metal oxidation and corrosion typically rely on superficial passivation of the metal with toxic materials such as chromic acid, sacrificial coatings (e.g., zinc or galvanized coatings), electroplated metals, polymeric coatings or related efforts that produce a protective barrier between the reactive metal surface and water. As another example, light oil treatments have also been used to protect metal surfaces.
Implementation of such strategies usually produces an inflexible anti-corrosion barrier on metal surfaces, and once applied, its removal may be difficult or impossible. In addition, the removal of such coatings can generate potentially hazardous waste materials. For those situations where micro-mechanical or circuit-based devices display corrosion tendencies, aggressive industrial anti-corrosion methods may be totally unsuitable and physically damaging. Thus, there is a significant need for new, simple-to-execute anti-corrosion barrier possibilities.